1. Field of the Invention
The present invention relates to the manufacture and assembly of structures, particularly those structures having an outer layer or skin secured to or supported by a sub-structure and where the outer layer is required to conform within close tolerances to a predetermined profile.
2. Discussion of Prior Art
It is often desirable to assemble such structures by first providing the sub-structure, or skeletal framework, and then attaching panels to the sub-structure to form the outer layer or skin.
This type of structure is common in the design and manufacture of aircraft where light-weight, high strength structures are required. In this industry it is additionally necessary to ensure that all the parts of the structure are manufactured within tight tolerances and furthermore that the parts fit together so that the assembled structure meets stringent accuracy requirements.
Any out of tolerance parts or inaccurately fitted parts will cause the adjacent parts being assembled to be fitted out of their required place in the structure, rendering the structure unacceptable. It is also important that adjacent parts of the outer skin do not have a step between them so that the constituent panels and skins of the outer layer of the aircraft structure are flush with one another. Failing to provide a substantially smooth aircraft outer layer can result in unwanted aerodynamic effects such as increased drag or turbulence.
To meet the strict tolerances required in aircraft construction for example the underlying sub-structure may be made from machinable aluminium or titanium. The sub-structure may then be machined as necessary to allow outer skins or panels to be fitted to it without adjacent skins or panels having a step between them. This method is not desirable as any machining errors may cause the whole sub-structure to fail a quality assurance inspection and be rejected with consequent cost and time penalties. Additionally underlying sub-structures are increasingly being made from lightweight composite materials such as carbon fibre reinforced plastics (CFRP) and these materials are not readily machinable.
A method of producing structures to high accuracy requirements is known, and can be used with sub-structures made of either metal or CFRP. In this method, the surfaces of sub-structure to which panels are to be attached are coated with a filled, two component liquid adhesive material, with aluminium added to it. The liquid adhesive is cured on the sub-structure, and is then machined to a desired thickness before the panels or skins are fixed to the sub-structure. The cured adhesive may be machined to different thicknesses at different locations on the sub-structure so that, when the panels or skins are fixed to it there is substantially no step between adjacent panels or skins.
Whilst this method produces structures having profiles with acceptable accuracy, it has several disadvantages. Adhesive of this type is a viscous liquid which must be applied carefully to the sub-structure by hand using a spatula, so that it is distributed reasonably evenly with the desired thickness and without creating air bubbles in the adhesive. Too much adhesive will result in a longer wait for curing and more time spent in machining than necessary. Adhesive of this type is difficult to apply in desired quantities because of its viscosity and furthermore, there are health and safety implications associated with its use. Personnel must be trained to use such adhesive and must be careful when applying it to the sub-structure. Also special tooling must be manufactured, tailor made for each area to be panelled, to prevent the liquid adhesive from spreading to areas where it is not required, and to give guidance as to the thickness of the adhesive being applied. Because of the nature of this type of adhesive, the tooling must be coated with a release agent before use and cleaned thoroughly after use. Repeated exposure to this coating and cleaning process causes the tooling to deteriorate rapidly after a relatively low number of uses, resulting in time lost and expense in manufacturing and fitting replacement tooling.
Furthermore, it is usually desirable to ensure that a joint between a substructure and an outer skin is both insulated and sealed, to prevent electrical current and liquids respectively from flowing through the joint. In the event of a lightning strike, current flowing from the outer skin through the joint to the substructure can damage the integrity of the joint, and thereby compromise the safety of the aircraft. Liquid flowing through the joint can cause corrosion, which may weaken the joint and/or the adjacent substructure and/or the outer skin.
Known methods of producing structures involve bonding a resin-impregnated glass fibre cloth to the inner surface of the outer skin, to provide an insulative layer between the outer skin and the substructure. The substructure, or a layer formed thereon as described above, is then machined to the required tolerance, and finally sealant is applied between the substructure and the glass fibre cloth before the substructure and the outer skin are bonded or bolted together.
This method has several drawbacks. Firstly, the glass fibre cloth, having different material characteristics to the outer skin, can expand at a different rate to the outer skin, which may be carbon fibre for example. Such differential expansion may cause delamination or distortion of the outer skin, both of which have serious consequences for aircraft. Secondly, the sealant, which is generally obtainable as a viscous liquid, is often squashed out of the joint during assembly to an extent that the joint is rendered permeable. Thirdly, the machining of the substructure or the layer formed thereon to obtain the required tolerances is time consuming and has the difficulties and disadvantages described in detail above.